Optical parametric oscillators (OPOs) use parametric frequency conversion in a non-linear crystal to convert light at a fixed short wavelength to tunable outputs at longer wavelengths. More specifically, OPOs take a high-energy, i.e. short wavelength, photon and divide its energy between two newly generated lower-energy, i.e. longer wavelength, photons. The input photon is called the pump and the two outputs are typically called the signal and idler wavelengths, by usual convention the signal wavelength being the shorter of the two wavelengths. For an OPO to work the cavity in which it is located will usually be resonant and have a low loss at the signal and/or idler wavelength(s) and the non-linear crystal has to be phase-matched to efficiently generate light at the correct wavelength.
OPOs are flexible sources of coherent radiation that can be tuned over substantial bandwidths in the ultraviolet, visible and infrared spectral regions. Examples of OPOs are described in the articles “Continuous-wave, singly-resonant intra-cavity optical parametric oscillator based on periodically-poled LiNbO3”, by Turnbull et al, Electronics Letters 33(21), pages 1817-1818 (1997); “Widely Tunable all-solid-state optical parametric oscillator for the visible and near infrared” by Cui et al, Optics Letters 18(2), pages 122-124 (1993), and “Tunable ultraviolet optical parametric oscillator for differential absorption lidar measurements of tropospheric ozone” by Fix et al, Applied Physics B 75(2-3), pages 153-163 (2002).
OPOs have been operated on many timescales from the femtosecond pulse to the true continuous-wave. In the case of the latter, the advent of new non-linear materials, in particular periodically-poled non-linear materials, has resulted in these devices becoming practical sources. Periodically poled OPOs comprise non-linear material in which the crystal domain structure is periodically reversed, as shown in FIG. 1. By varying the periodicity of the domain pattern in the crystal, the exact signal and idler wavelengths, which are phase matched to a given pump wavelength, can be changed. In practice, the domains can be periodically reversed by applying a high voltage across the crystal through a patterned electrode.
Despite the advent of periodically poled non-linear materials, problems with the practicality of OPOs still exist. A particular problem, which restricts development of compact/miniature devices, is that substantial pump powers are required for the parametric oscillator to reach threshold. One solution to the high threshold problem is to put the optical parametric oscillator within the cavity of the pump laser. This type of device is known as an intra-cavity optical parametric oscillator. Such a device has been described by a number of authors, see in particular “Continuous-wave, singly-resonant, inter-cavity parametric oscillator” by Colville et al, Optics Letters 22(2), pages 75-77 (1997); “Optical parametric devices and processes” by Ebrahimzadeh, JOSA B 16(9), page 1477 (1999); “Parametric generation of tunable light from continuous-wave to femtosecond pulses” by Dunn et al, Science 286(5444), pages 1513-1517 (1999), and “Internal optical parametric oscillators”, by Oshman et al, IEEE, J. Quantum Electronics QE-4, pages 491-502 (1968).
FIG. 2 shows an example of a known intracavity optical parametric oscillator. This has a laser pump arrangement having a semiconductor laser diode 10, a lens 12 and a gain medium 14, into which radiation from the semiconductor laser diode 10 is directed. The lens 12 is provided for optimally matching the spatial profile of the radiation from the laser diode 10 to the mode size, preferably the fundamental mode, of the radiation in the gain chip 14. As a specific example, the laser gain medium 14 is neodymium:vanadate, and the semi-conductor laser diode 10 is adapted to deliver one watt of optical power at 809 nanometers, which is a strong absorption feature of neodymium:vanadate.
On a back surface of the gain medium 14, and integral with it, is a reflective material that defines a first mirror 16. Opposite the gain medium 14 is a second reflective surface 18. Between the laser gain medium 14 and the second reflective surface 18, and along an optical axis thereof, are in sequence a lens 20, a beam splitter 22 and a non-linear material 24, in this case a crystal of periodically poled lithium niobate (PPLN) that is about 50 mm long and has a grating period of 29.3 microns. The purpose of the lens 20 is to enable the appropriate mode sizes to be obtained in the laser gain medium 14 and the non-linear material 24, when used in association with the first and second mirrors 16 and 18. Off the main optical axis is provided a third mirror 26, which is positioned so that light reflected from the beam splitter 22 is directed onto it.
Each of the first and second mirrors 16 and 18 is highly reflective at the wavelength of the light, the pump radiation, emitted from the laser gain medium 14. The beam splitter 22 is highly transmissive at the pump radiation so that it allows light emitted from the gain medium 14 to pass through it and into the non-linear material 24, whilst at the same time is highly reflective to down converted waves emitted from the non-linear material 24 so as to reflect such radiation either onto the third mirror 26 or back into the non-linear material 24. It will be appreciated that a number of combinations of reflectivities at the signal and idler wavelengths of the second and third mirrors exist depending on which or both are the resonant waves. In this case, the second mirror 18 is wholly reflective at the signal wavelength and wholly transmissive at the idler wavelength so that an output can be gained. The third mirror is wholly reflective to down converted light emitted from the non-linear material.
As will be appreciated, the arrangement of FIG. 2 has two coupled cavities, namely a laser pump cavity defined by the optical path between the first and second mirrors 16 and 18, in which the non-linear element 24 is located along with the gain medium of the pump laser 14 itself, and a second cavity, defined by the optical path between the second and third mirrors 18 and 26, that is associated with the wave of the down converted coherent radiation generated by the non-linear material 24.
When the arrangement of FIG. 2 is used, stimulation of the non-linear material 24 by the pump laser 14 causes an optical parametric down conversion process to start and so generates a pair of signal and idler waves. In practice it has been found that the short-term stability (×10−6-×10−3 seconds) of the intra-cavity pump field is poor when this down conversion process is present. This can be seen in FIG. 3, which shows the temporal stability recorded by a photodiode that has a response time that is significantly less than the oscillation period. Also shown in FIG. 3 is the intra-cavity pump field stability when the down conversion process provided by the optical parametric oscillator is inhibited, for example, by placing a shutter between the beam splitter 22 and the third mirror 26. It is seen that in this case the pump field exhibits stable operation. Hence, the inclusion of the intra-cavity parametric oscillator within the laser cavity significantly modifies the dynamics of the intra-cavity pump field in the form of relaxation oscillation behaviour, most notably the period and decay time of these oscillations.
As is well known, the occurrence of relaxation oscillations can prove severely detrimental to the operation of an optical parametric oscillator as a stable source in terms of both amplitude and frequency stability of the coherent radiation generated. This is discussed in the articles “Continuous-wave intracavity optical parametric oscillators: an analysis of power characteristics”, by Turnbull et al, Applied Physics B 66, pages 701-710 (1998) and “Transient dynamics of CW intracavity singly resonant optical parametric oscillators”, by Turnbull et al, IEEE, Journal of Quantum Electronics 35(11), pages 1666-1672 (1999).
Relaxation oscillations are widely known in laser devices. They occur in particular when the upper laser level lifetime exceeds the decay time of the coherent radiation in the passive cavity of the laser. For example, such relaxations are widely known in the case of neodymium lasers and semi-conductor lasers, see “Output fluctuations of CW-pumped Nd: YAG lasers”, by Koechner, IEEE Journal of Quantum Electronics QE-8(7), pages 656-661 (1972), and “Relaxation oscillations in quasi-single-mode semiconductor lasers”, by Zaibel et al, IEEE Journal of Quantum Electronics 3(9), pages 2081-2086 (1994). However, in the case of intra-cavity optical parametric oscillators, where two coupled cavities are involved in the dynamics of the device, it has been shown, both experimentally and theoretically, that the effects of relaxation oscillations are particularly severe; see previous references to Turnbull et al. These relaxation oscillations can be triggered by many different mechanisms, for example thermal effects in the non-linear medium and interferometric feedback.